The present invention relates to an improvement of the apparatus for longitudinally and transversely trimming articles moldingly formed on the plastic sheet as disclosed in the copending application Ser. No. 642,980.
Various articles are formed by feeding a heated and softened thermoplastic web between a pair of male and female dies which are normally separated and by bringing said dies engaged together. Said dies may be subjected to raised or reduced pressure as occasion demands. Rectangular articles such as bath tubs, refrigerator inner rooms and the like have been manufactured according to such method. For instance, the freezing room and the cooling room of the refrigerator, the former being smaller in general than the latter, are preferably formed by one shot of the forming machine operation for the convenience of assembly.
Unnecessary portion of the plastic sheet surrounding the fomred articles must be trimmed. Conventionally, a set of upper and lower frames, each comprising four or more blades so arranged as to correspond to the four or more lines of the plastic sheet to be severed have been used so that transverse and longitudinal trimmings are made in one shot by relatively moving said set of frame blades in vertical direction to engage with each other. According to such conventional trimming method, the cut-off waste portion of the plastic sheet is in one piece like as a picture frame held by a pair of clamp link chains along opposite side edges thereof so as to efficiently recover much amount of waste.
According to such conventional trimming device, however, every time when size of the article to be formed is varied, the set of trimming frame blades must be exchanged which necessitates many sets of blades of various sizes to be stored, which is space and time consuming. When the trimming device is combined with the moldingly forming machine, which is in turn usually combined with an extruder feeding plastic web thereto, so as to be intermittently operable in synchronized relation therewith, said forming machine and further said extruder if it is combined therewith must be stopped during such time consuming blade exchange work.
In order to avoid said defects, it was proposed in said copending application to provide a trimming apparatus comprising a set of longitudinally shiftable transversely movable pointed tip knives and a set of transversely shiftable longitidially movable pointed tip knives, respectively normally held in retracted position but movable to be brought in forwarded position where each pointed tip of said knives engages with the plastic sheet, said knives being so arranged that the distances between the concerned knives may be adjusted by correspondingly shifting and the plastic sheet may be trimmed by moving said knives which have been brought in said forwarded position.
It is not always necessary but actually preferable to provide a set of longitudinally shiftable transversely extended holder members and a set of transversely shiftable longitudinally extended holder members, respectively normally held in retracted position but movable so as to be brought in forwarded position where each member hold the plastic sheet along the respective line to be severed to cooperate with the respective counterpart knife, said holder members being so arranged that the distances between the concerned holder member may be adjusted by correspondingly shifting.
When the knives for longitudinal and transverse severing are separately provided, however, the waste sheet portion is to be divided in a plurality of pieces, which is not preferable by the reason referred to above.
In order to avoid such defects, it was disclosed in said copending application to provide longitudinally and transversely extending bent ridges formed on the plastic sheet concurrently with forming the articles so that said pointed knives brought in the respective forwarded position may sever the plastic sheet except said ridges to which each pointed tip of said knives cannot reach.
However, it was found difficult in such trimming apparatus to sever the sheet along the correct lines due to that the sheet is still plastic. The severed line is not only often deviated from the line to be severed but also not straight and zig-zagged or curved more or less.